The guitar introduction to the studio version of "The Boxer" on the Bridge Over Troubled Water album is played by a different guitar than the rest of the song.   The intro guitar (which also continues throughout the song) is played by Fred Carter Jr. and is tuned to an alternate tuning rather than standard pitch (EADGBE).   There is a tab for "The Boxer" on Rich Kent's web site which has this introduction transcribed for a guitar tuned to standard pitch, however that is not the way it was played in the studio.   When Paul Simon plays "The Boxer" in concert, he doesn't play the studio version introduction.

(2) "April Come She Will"

There are different capo positions for the various album recordings of this song:

Paul Simon Songbook -- no capo, tuned down 1/4 step
Sounds of Silence -- capo 2nd fret, tuned down 1/4 step
Concert in the Park -- capo 3rd fret, standard tuning

(3) "A Heart in New York"

Although this is not a Paul Simon song, Art Garfunkel does a beautiful rendition of it on the Simon and Garfunkel Concert in Central Park album.   There are at least 2-3 guitars on this recording (and Paul Simon is not playing any of them).   Here is a nice tab for solo guitar:

              A Heart in New York